Download Subduction erosion along the Middle America convergent margin

letters to nature
stabilized by polyvalent polymer±colloid interactions. Roughly
spherical aggregates of similar dimensions (,60 nm) were obtained
from 6-nm analogues of Thy-Au and polymer 1 (see Supplementary
Information), demonstrating the applicability of our polymermediated approach to the assembly of different sized nanoparticle
subunits.
Self-assembly processes are governed by a balance of entropic and
enthalpic effects, making them very temperature dependent. This
temperature dependence is manifested by more ef®cient recognition processes at lower temperatures21, which would be expected to
yield larger aggregate structures. Investigations of temperature
effects on the preparation of the aggregates yielded results consistent
with this prediction. TEM micrographs of the precipitate formed at
-20 8C revealed the formation of microscale (0.5±1 mm) discrete
spherical particles (Fig. 5b), consisting of (6±50) ´ 105 individual
Thy-Au units. These microscale particles are among the most
complex synthetic self-assembled structures known, demonstrating
the thermal control of aggregate size using the `bricks and mortar'
methodology.
In addition to controlling the size of the aggregates, temperature
strongly affects the morphology of the resulting ensembles. At 10 8C,
networks were formed (Fig. 5c), as opposed to the discrete structures observed at higher and lower temperatures. This suggests that
network formation is an intermediate process in the formation of
the giant assemblies at -20 8C. The individual assemblies within
these networks remained spherical, although their sizes are more
highly dispersed. We are currently investigating methods aimed at
achieving control over the size and the geometry of these networks,
which would allow the fabrication of rod- and wire-like structures
for incorporation in nanoscale constructs (see Supplementary
Information).
We have demonstrated a polymer-mediated strategy for the selfassembly of nanoparticles into structured ensembles. Discrete
microspheres formed using this methodology are truly multi-scale
constructs, highly ordered on the molecular, nanometre and micrometre scale. This method has also been shown to generate network
structures, which can serve as precursors to other shapes and sizes of
nanoparticle assembly. The degree of ordering and the control of
particle size and shape, coupled with the inherent modularity of the
`bricks and mortar' colloid-polymer self-assembly process, represent a powerful and general strategy for the creation of highly
structured multifunctional materials and devices.
M
Received 8 July 1999; accepted 10 February 2000.
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Acknowledgements
This work was supported by the NSF. V.M.R. also acknowledges support from the
Petroleum Research Fund of the ACS, the Center for University Research in Polymers at
the University of Massachusetts, the Alfred P. Sloan Foundation, Research Corporation,
and the Camille and Henry Dreyfus Foundation.
Correspondence and requests for materials should be addressed to V.M.R.
(e-mail: [email protected]).
.................................................................
Subduction erosion along the
Middle America convergent margin
C. R. Ranero & R. von Huene
GEOMAR, Wischhofstrasse 1-3, Kiel 24148, Germany
..............................................................................................................................................
`Subduction erosion' has been invoked to explain material missing
from some continents along convergent margins1. It has been
suggested that this form of tectonic erosion removes continental
material at the front of the margin or along the underside of the
upper (continental) plate2±4. Frontal erosion is interpreted from
disrupted topography at the base of a slope and is most evident in
the wake of subducting seamounts5,6. In contrast, structures
resulting from erosion at the base of a continental plate are
seldom recognized in seismic re¯ection images because such
images typically have poor resolution at distances greater than
, 5 km from the trench axis. Basal erosion from seamounts and
ridges has been inferred7,8, but few large subducted bodiesÐlet
alone the eroded base of the upper plateÐare imaged convincingly. From seismic images we identify here two mechanisms of
basal erosion: erosion by seamount tunnelling and removal of
large rock lenses of a distending upper plate. Seismic crosssections from Costa Rica to Nicaragua indicate that erosion may
extend along much of the Middle America convergent margin.
From Costa Rica to Nicaragua the Paci®c continental margin is
formed by a wedge-shaped body of igneous rock (margin wedge),
covered by 1±2 km of slope sediment and fronted by a small
sediment prism9±11. The facing ocean plate has a variable structure;
there is thick crust at the Cocos ridge12, the plate is ,40% covered by
seamounts off central Costa Rica, and has a smooth morphology off
the northwest Nicoya peninsula (Fig. 1). Off Nicaragua, the sub-
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ducting ocean crust is thin (,5 km) and cut by many normal faults
with ,0.5-km displacement11,13,14. Variations of crustal structure
and sea¯oor morphology of the subducting plate correspond with
regional and local changes in upper-plate thickness along the
margin. On a regional scale, the ocean crust thins and deepens
towards Nicaragua, whereas the corresponding continental margin
wedge thickens (Fig. 2). Where the buoyant Cocos ridge subducts,
the margin wedge is thinner than opposite the adjacent seamount
segment (Fig. 2). Facing the seamount segment the margin is
thinner than to the northwest and is thinnest at a large embayment
off the southeast Nicoya peninsula (Figs 1 and 2). The margin wedge
thickens off the northwest Nicoya peninsula where the featureless
sea ¯oor subducts, and is even thicker off Nicaragua, where deep sea
¯oor and thin crust subduct.
The thin margin wedge (Fig. 2) and indented continental slope
(Fig. 1) opposite the seamounts and the Cocos ridge suggest that
repeated seamount subduction is an effective agent of upper-plate
erosion. Opposite the seamount segment the slope has several
grooves parallel to the convergence vector (G1 to G4, Fig. 1). The
grooves mark the path of subducting seamounts, and the local uplift
at the end of G1, G2 and G4 indicates that three seamounts are
underthrusted beneath the slope14. The grooves indicate missing
upper-plate material associated with seamount subduction. Two
upper-plate areas are eroded during seamount subduction. Seamount impact erodes the margin front producing re-entrant
structures at the base of the continental slope (for example, G1
and G4, Figs 1 and 3a). Landward, in the 30±40 km of the margin
upslope of the frontal prism, the margin wedge remains semicoherent as seamount underthrusting uplifts, fractures and thins the
upper plate. The resulting rough margin-wedge upper surface has
relief locally exceeding 0.5 km (Fig. 3a, b). Discontinuous strata are
locally folded parallel to the surface (Fig. 3b), and undeformed
younger strata smoothes this topography. The rough surface and a
disrupted canyon system are roughly coincident. Seismic images
from areas where seamounts have subducted (for example, Fig 3a)
and along the grooves of subducting seamounts14 indicate that only
the upper part of the slope sediment slides at the oversteepened
slope above subducting seamounts. Although some of the slumped
sediment is missing, it is not enough to explain the grooves. The
groove G3 penetrates the most into the margin (Fig. 1) where it is a
deep trough collecting sediment delivered by shallow canyons (Fig.
3a). Although the groove is partially ®lled with recent sediment, the
anomalous depth indicates missing material and erosion at the base
of the upper plate.
Where seamounts no longer disrupt the front of the margin
wedge but tunnel beneath it, we have imaged active basal erosion.
Above a seamount about 2 km high, 0.5±0.7 km of rock from the
base of a 2-km-thick margin wedge is missing (Fig. 4), which helps
to explain the grooves and the thin margin wedge at the embayments. It has been proposed that uplift of the decollement over a
Figure 1 Bathymetry and tracks of seismic data off Costa Rica and Nicaragua. a,
Multibeam hydrosweep bathymetry off Costa Rica with tracks of Sonne-81 and Shell
P1600 multichannel seismic re¯ection data. The sudden change in roughness from the
continental slope to the platform occurs where the multibeam bathymetry coverage ends.
Bathymetry in the platform is from low-resolution maps. Thick lines mark segments shown
in Figs 3 (lines 4 and 17) and 4, (line 6). Filled dots are earthquake epicentres17. Only
earthquakes deeper than 30 km are plotted beneath the mainland. The ocean plate has
three segments: the Cocos ridge with crust .12 km thick12, the seamount segment with
topographic features 1±2.5 km tall and 10±20 km wide, and a smooth sea¯oor segment
off the northwest Nicoya peninsula. Subduction of seamounts is indicated by grooves in
the continental slope (G1±G4) and clusters of earthquakes beneath the continental slope
and shelf. Large embayments in the margin morphology indicate areas where seamount
subduction has eroded the upper plate14. b, Location of the transect along the ocean plate
(dashed line) and of the cross-sections T1±T7 used in Fig. 2. Cross-section T2 is a
prestack depth migration. The other cross-sections are constrained by wide-angle and
coincident near-vertical seismic data. Box off northwest Nicoya peninsula marks the area
of a 3D seismic data set19.
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seamount might create a ¯uid pressure gradient that drains ¯uid
from the seamount ¯anks and concentrates it at the crest, thereby
promoting hydrofracturing and low friction8. However, the pervasive fracturing observed in the bathymetry at the crest of domes
above subducting seamounts (Fig. 3a) provides ¯uids conduits for
venting that probably relieve pressure. The increase in normal stress
across the plate interface resulting from the accommodation of the
seamount15 in addition to the roughness of the volcano probably
facilitates abrasion along the seamount subduction path16. Earthquakes of magnitude greater than 4 at the front of the margin17 are
located near the subducting seamounts and have strike-slip rather
than thrust focal mechanisms, indicating a relatively high friction
between seamounts and upper plate. Where the upper plate is .6±
8 km thick, seamounts uplift but do not break up the margin wedge
(Fig. 3a), so its upper surface is smooth, disrupted only by landward
dipping normal faults that extend into the overlying strata (Fig. 3c).
On several seismic lines, plate-boundary re¯ections deeper than
, 6 km bifurcate and surround megalenses (10±15 km long, 1.5±
2 km high) whose origin is puzzling (Fig. 3a and d). Similar
structures are observed off the northwest Nicoya peninsula18,19.
The megalenses occur in areas where the upper plate extends by
normal faulting, and could represent material transferred from the
lower to the upper plate (for example, underplated sediment or a
sheared-off seamount) or alternatively margin-wedge rock transferred to the lower plate. The size in two dimensions of some
megalenses is equivalent to a seamount (Fig. 3d), so if they had been
transferred from lower to upper plate the uplift should resemble
that of currently underthrusting seamounts. However, seismics and
bathymetry (Fig. 3d) show no large doming, but rather a relatively
stable slope with a canyon system, and also a less disrupted margin
wedge and slope sediment than along seamount paths, inconsistent
with a subducting or sheared-off seamount. Underplating might
require more time and thus the slope may have stabilized, but
the , 2-km-thick lens would require a large uplift of the margin
wedge (see, for example, Fig. 4) which is not observed (Fig. 3d).
Thus, the megalenses are probably being removed from the
upper plate. Further support for erosion comes from the style
of normal faulting. Some re¯ections from the landward-dipping
fabric of events within the margin wedge project to offsets at the
top, indicating that normal faults cut deep into the upper plate.
Landward-dipping, deep-penetrating normal faulting has also
been described in the middle slope off northwest Nicoya18. On
line 4, fault dip decreases and fault throw grows trenchwards
(Fig. 3e), with the largest fault throws seaward of the megalens
where the margin wedge is thinner (Fig. 3d). Extension, thinning
and collapse of the margin seaward of the megalens is the
response to the material transfer. The megalenses occur at
depths at which temperatures calculated from depth to
bottom-simulating re¯ectors indicate that the smectite±illite
transition has occurred, and the plate interface is entering the
zone of stick-slip behaviour20. Thus, the lenses could be associated with active erosion in an environment of increasing
friction. This mechanism has no apparent relation to seamount
subduction, and might be active in other areas where relatively
smooth ocean ¯oor is subducted. Beneath the upper slope, the
upper boundary of the megalenses is a linear splay from the
plate-boundary interface at about 10±11 km depth. The landward-dipping upper-boundary re¯ections have a similar or
slightly shallower dip than linear re¯ections above, within the
margin wedge, which might indicate that megalenses form using
pre-existing planes of weakness. Movement along a pre-existing
weak interface may be favoured when ¯uids expelled from
subducting sediment reach the structure and reduce friction,
which detaches a lens of margin wedge rock. Seaward, beneath
the middle slope, the landward-dipping upper boundary of the
megalens merges with a subhorizontal group of re¯ections (Fig.
3d) which might indicate a lithological boundary within the
margin wedge.
Tectonic erosion has been inferred from long-term subsidence of
the continental slope off Nicaragua, and the steep slope with scars of
numerous landslides indicates that erosion is probably currently
active11,14. Thus, tectonic erosion may not be limited to the region
where thick crust and rough sea ¯oor subducts, but could extend
along much of the Middle America trench. However, thinning and
deepening of the ocean crust, and the smaller sea¯oor relief towards
Nicaragua, correspond to changes in tectonism along the plate
boundary. Closely spaced normal faulting of the ocean plate off
Nicaragua creates a `washboard' topography. Subduction of this
type of morphology has been proposed as an erosional
mechanism21. However, the upper plate off Nicaragua is thick
relative to that of northwest Nicoya; this indicates that the `washboard' topography is either not an ef®cient mechanism of basal
erosion or that erosion there is limited to the frontal ,10 km of the
upper plate.
In addition to the erosional mechanisms discussed above, the
subduction of the entire ocean pelagic-hemipelagic section indicates
Figure 2 Comparison of continental framework thickness with the structure of the facing
ocean plate. a, Cross-sectional area (km2) of the continental framework (margin wedge) of
the upper plate from Costa Rica to Nicaragua. Values T1±T7 are the area of the frontal
50 km of margin wedge calculated along the transects shown in Fig. 1. b, Crustal
structure of the ocean plate along the transect in Fig. 1b. Downward-pointing arrows with
numbers indicate crosspoints of transect with seismic tracks used to constrain the crustal
structure. Double-headed arrows indicate extent of seismic lines along the transect. Data
from refs 12, 13, 23 and this study.
750
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letters to nature
Figure 3 Prestack depth migration of Sonne-81 lines 17 and 4 projected on bathymetry
perspectives. The iterative migration procedure uses velocities constrained with focusing
analyses and common re¯ection point gathers24. Wide-angle data guided velocity picking
at depths greater than ,5 km. Resolution at ,10 km is ,0.5 km. a, Line 17 is 55 km
long from the toe to the upper continental slope. The line is where seamounts have
recently subducted. Note the small frontal prism, the continuity of plate boundary
re¯ections and the distinct top of the margin wedge. The frontal ,40 km of the margin
have a rough margin-wedge top (inset b) produced by underthrusting of seamounts like
those shown in the bathymetry. Arrows point to grooves in the bathymetry (G1±G3)
produced by subducting seamounts, and the doming up of the sea ¯oor indicates the
location of two of those seamounts. Where the margin wedge is .6±8 km thick its top is
NATURE | VOL 404 | 13 APRIL 2000 | www.nature.com
smooth, only cut by normal faulting (inset c). Beneath the upper slope, plate-boundary
re¯ections bifurcate and surround a rock megalens. d, Line 4 is 58 km long from the toe to
the upper continental slope. The top of the margin wedge is smooth beneath the upper
slope, cut by small throw normal faults (inset e). Fault throw grows and fault dip decreases
downslope and the top of the margin wedge becomes rougher. The canyons in the upper
and middle slope indicate a relatively stable environment where no seamount has recently
underthrusted. Thrust faulting occurs only at the small frontal prism. The frontal prism is a
distinct morphological unit, tectonically active as indicated by the disruption of the slope
drainage system. Plate-boundary re¯ections bifurcate at , 6 km depth and surround a
rock megalens.
© 2000 Macmillan Magazines Ltd
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letters to nature
22. Aubouin, J. et al. Leg 84 of the deep drilling project:subduction without accretion: Middle America
Trench off Guatemala. Nature 297, 458±460 (1982).
23. Ye, S. et al. Crustal structure of the subduction zone off Costa Rica derived from OBS refraction and
wide-angle re¯ection seismic data. Tectonics 15, 1006±1021 (1996).
24. Mackay, S. & Abma, R. Depth focusing analysis using wavefront-curvature criterion. Geophysics 58,
1148±1156 (1993).
Acknowledgements
We thank T. Reston for comments on the manuscript. SONNE-81 seismic re¯ection data
were collected by the BGR.
Figure 4 Active tectonic erosion by seamount tunnelling. The seismic image is a prestack
depth migration of a segment of Sonne-81 line 6 over a subducting seamount. Dots mark
the plate boundary, with black dots delineating the seamount ¯anks. This segment of the
seismic line is parallel to the continental margin, and shows lateral variations in margin
wedge thickness. The margin wedge is 0.5±0.7 km thinner above the subducting
seamount. Extension of the upper plate by uplift above the seamount is too small to
explain the thinning. Thus, thinning is probably due to tectonic erosion produced by
mechanical wearing of the upper plate. The location of this cross-section is shown on
Figs 1 and 3.
that hydrofracturing and piecemeal stoping2,4,8 (an underground
mining process) of the base of the upper plate by overpressured
¯uids may also contribute to upper-plate thinning. A body of PlioPleistocene sediment at the front of the margin from southern Costa
Rica to Guatemala is limited to a prism less than 10 km wide11,14,22.
Although underplating at the rear of the frontal prism might be
active or an older relatively small body of accreted material may be
present19, accretion of sediment must be limited in space and time.
Subduction erosion at present dominates processes off Costa Rica,
M
and probably also extends into the Nicaraguan margin.
Received 6 August 1999; accepted 22 February 2000.
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Correspondence and requests for materials should be addressed to C.R.R.
(e-mail: [email protected]).
.................................................................
Quantitative evidence for global
amphibian population declines
Jeff. E. Houlahan*, C. Scott Findlay*², Benedikt R. Schmidt³,
Andrea H. Meyer§ & Sergius L. Kuzmink
* Ottawa-Carleton Institute of Biology, University of Ottawa, 30 Marie Curie,
Ottawa, Ontario K1N 6N5, Canada
² Institute of Environment, University of Ottawa, 555 King Edward Street,
Ottawa, Ontario K1N 6N5, Canada
³ Zoologisches Institut, University of ZuÈrich, Winterthurerstrasse 190,
CH-8057 ZuÈrich, Switzerland
§ Swiss Federal Statistical Of®ce, Sektion Hochschulen und Wissenschaft,
Espace de l'Europe 10, CH-2010, NeuchaÃtel, Switzerland
k Institute of Ecology and Evolution, Russian Academy of Sciences,
Moscow 117071, Russia
..............................................................................................................................................
Although there is growing concern that amphibian populations
are declining globally1±3, much of the supporting evidence is
either anecdotal4,5 or derived from short-term studies at small
geographical scales6±8. This raises questions not only about the
dif®culty of detecting temporal trends in populations which are
notoriously variable9,10, but also about the validity of inferring
global trends from local or regional studies11,12. Here we use data
from 936 populations to assess large-scale temporal and spatial
variations in amphibian population trends. On a global scale, our
results indicate relatively rapid declines from the late 1950s/early
1960s to the late 1960s, followed by a reduced rate of decline to the
present. Amphibian population trends during the 1960s were
negative in western Europe (including the United Kingdom) and
North America, but only the latter populations showed declines
from the 1970s to the late 1990s. These results suggest that while
large-scale trends show considerable geographical and temporal
variability, amphibian populations are in fact decliningÐand
that this decline has been happening for several decades.
Over the past several decades, there have been reports of
catastrophic declines and extirpations of amphibian species in
Australia, South and Central America13±16, and in high-altitude
regions of the western United States17. Ancillary evidence has
pointed to several possible underlying causes, including changes
in local climate18, acid precipitation19, disease20,21, increased UV-B
irradiation22 or various combinations thereof23,24. Because these
reports usually document the disappearances of species from
small geographical regions, extrapolations to `global' amphibian
declines are tenuous at best. To address this problem of restricted
geographical coverage, we have analysed 936 amphibian population
data sets collected from journal publications and technical reports,
as well as unpublished data provided by herpetologists from around
the world. Every effort was made to be exhaustive, and we believe
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NATURE | VOL 404 | 13 APRIL 2000 | www.nature.com